Cell for electrolysis of hydrochloric acid



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CELL FOR ELECTROLYSIS OF HYDROGHLORIC ACID Filed Deo. 2l 1960 2Sheets-Sheet 2 Hlrullll L INVENTOR W mow@ MESSNER BY oRoNzlo DE NORAATTORNEY United States Patent 3,242,065 CELL FR ELECTRLYSS F HYDRO-CHLRIC ACID @ronzio Ee Nora and Georg Messner, Milan, italy, as-

signors to @ronzio De Nora Impianti Elettrochimici, Miian, italy, acorporation of Italy Filed Dec. 21, 1960, Ser. No. '77,402 Claims. (Cl.204--256) This invention relates to improvements in electrolytic cellsfor the production of chlorine by the electrolysis of hydrochloric acid.

In a number of chemical operations such as chlorination of organiccompounds, hydrochloric acid is formed as a by-product. Since the needfor hydrochloric acid at the place of its production is negligible andthere is usually no immediate market for it, the hydrochloric acidpresents a disposal problem. Y -It can not be dumped in streams withoutneutralization because of stream pollution, and to neutralize the acidbefore dumping is prohibitive because of the cost.

It had been proposed to regenerate chlorine from the hydrochloric acidthereby eliminating the disposal problem and recovering the expensivechlorine otherwise lost as hydrochloric acid. The oxidation of thehydrochloric acid with oxygen in the presence of a catalyst or withnitrogen peroxide has been .attempted but equipment and operations costsfor these operations are too -high with reference to the value of thechlorine recovered.

Therefore, industry has turned to the electrolysis of hydrochloric acidto form chlorine and hydrogen as a means of overcoming the problems. Theequipment necessary for the operation is simple and the installationcost is low. Moreover, the process is flexible because the load of theelectrolytic cells can be varied above or below Irated capacity to allowfor change in amounts of hydrochloric acid produced as a by-product.

One of the better cells for the electrolytic process is described in theF.I.A.T. Final Report No. 832 issued by the Ofce of Military Governmentfor Germany (US.) on June 16, 1946. This cell has a filter press typeassembly and consists of a number of individual or unit cells clampedtogether to form a liquid tight assembly. Each unit cell consists of anouter Haveg frame which is covered with a corrosion-resistant material.Into this frame a bipolar graphite electrode is fastened. The bipolarelectrode is grooved ou the cathode side from bottom to top to alloweasy passage of hydrogen gas to the top and is made smooth on the otherside, and a diaphragm is placed over the grooves. When the frames areplaced in the electrolytic cell assembly, there is a space between thesmooth anode side of the bipolar graphite electrode and the ldiaphragmin the next frame, and this space is filled with graphite lumps. Thelumps act as the anode and allow the free chlorine gas to rise to thetop. Since the anode is consumed during the process, more graphite lumpscan be added from holes in the top of the frame without disassemblingthe cells.

While the said cells function quite well, they have some defects due tothe use of graphite lumps as anodic material. The electrical contact ofthe graphite lumps with the smooth side of the graphite plates is notcomplete due to the spaces between the lumps, and there is a voltagedrop or loss in the unit cell. This loss of electrical energy raises thecell operating temperature to about 95 C. and makes the cell moresusceptible to corrosion. The graphite lumps may also cause slightbowing or buckling of the `diaphragm where the lumps contact thediaphragm and particularly near the bottom of the cell, due to thegreater weight of the graphite lumps. Due to the deformation of thediaphragm, some lumps will be closer to the corresponding graphitecathode and will cause a ICC concentration of the current at thosepoints. The current concentrationcauses local heating which furtherweakens the diaphragm which will eventually be destroyed.

An attempt to overcome the problems of the use of graphite lumps as an`anode has been made in the German patent application No. 1,054,430,published April 9, 1959. In this cell there is a gas-tight andcurrent-tight wall attached to the filter press frame members whichseparates the grooved graphite cathode and anode plates. The cathodesand anodes are attached by means of graphitic bolts passing through theseparating wall.. The graphite bolts hold the cathode and anode againsteach side of the separating wall and allow the current to pass throughthe cell from the anodes to the cathodes through the graphite bolts.Gaskets are used to separate the anode and cathode plates from theseparating walls and to prevent passage of the electrolyte from theanode to the cathode compartments and separate diaphragms are heldbetween each frame of the filter press assembly. T-he separating walls`are made of cast iron or steel coated with a nonconducting material,and act as a mechanical support `for the electrodes.

In order to replace the anode plate which is consumed in the process inthe cells of the said German application, it is necessary to disassemblethe cell units and also separately remove the diaphragm and the cathodeplate which is not consumed in the operation of the cell and thiscomplicates the disassembly and reassembly of the cell units. Moreover,the assembly and disassembly of the cell is further complicated by thegas separator and the diaphragm which are parts separate from the filterpress construction of the cell. During the disassembly particularly thediaphragm is susceptible to damage when removing it. When assembling thecell, it is difficult to position the gas separator and diaphragmexactly in place so that there is no leakage between the gas collectingchambers or distortion of the diaphragm, and each section of the anodeand cathode graphite plates and their corresponding gaskets must beseparately vassembled and held on each side of the separating wallswhile the graphite bolts which connect the anode and cathode plates arescrewed into place.

It is an object of this invention to provide an electrolytic unit cellfor the recovery of chlorine and hydrogen `from hydrochloric acid whichoperates with low electrical consumption and at low temperatures.

lt is another object of this invention to provide a simply constructedunit cell for the electrolysis of hydrochloric acid which will operatefor a period of years without the necessity for replacement of the anodeplates.

It is a further object of the invention to provide a unit cell for theelectrolysis of hydrochloric a-cid wherein the anode is bolted to thecathode which is formed integral with graphite plates attached into theindividual filter press frames for easy assembly and disassembly.

It is another object of this invention to provide a unit cell forelectrolysis of hydrochloric acid in which the diaphragm and the grooved'cathode plates do not have to be removed during assembly anddisassembly.

These and other objects and advantages of this invention will becomeapparent from the following detailed description.

The prior art disadvantages are overcome by the unique construction ofthe unit cell of the present invention described herein. The need for aseparating wall is eliminated by cementing grooved cathode platesdirectly to the frame. There is no need to remove the cathode as onlythe anodes are consumed during the operation of the cell. Therefore,assembly and disassembly of the cell is made easier by the novelconstruction described herein as only the anode need be removed. Also,since the cathode need not be removed during the changing of the anodeplates, the diaphragm covering the cathode plate is permanently cementedto the frame as it does not have to be removed. This prevents damage ordistortion of the diaphragm. Moreover, the electrolytic cell is easilyassembled because the gas separator is not a separate part, as in thesaid German application.

The grooved cathode plate may be made of graphite, coke or graphitecoated with coke. lf the cathode plate is graphite it should beactivated to reduce the cathode potential about 0.1 to 0.2 volt. Thegrooved graphite plate may be activated simply by applying theelectrical current to a complete electrolyser cell or installation inthe wrong direction fora few hours. In this manner the final cathodesact as anodes for a short time and are Slightly attacked by thetemporary chlorine development. The slight attack of chlorine activatesthe graphite plates and reduces the cathode potential.

The grooved cathode plates are preferably made of coked carbon orgraphite coated with coke. The use of a coke surface on the groovedcathode plates give a cathodic hydrogen overvoltage which is 0.2 to 0.3volt lower than the use of a graphite surface. This lower overvoltageresults in an electric energy saving of about The coke surface may beapplied to the grooved graphite cathode plates in any of the usualcoking processes. For example, the graphite plate may be impregnatedwith a liquid organic material such as a resin, tar or solution of sugarin water and then the impregnated graphite plate heated at temperaturesbetween 500 and 900 C.

Another method of forming a coke surface on the grooved graphite plateis to heat the said graphite plate at 500 C. for a few hours in anatmosphere containing carbon dioxide. The use of graphite plates as theanode rather than graphite lumps provides a better electrical contactand a lower voltage drop is obtained across the cell. The normal loss inthe graphite lump anode is about 400 millivolts while the usual loss inthe cell of the present invention is about 25 to 30 millivolts. Also,the operating temperature is lower and corrosion is reduced in the celldescribed herein giving a longer life to the cell. The diaphragms have alonger life because there is no local over-heating at spots of highercurrent density due to stress on the diaphragm because of lumps pressingagainst the diaphragm.

While the present electrolytic cell has to be disassembled to replacethe anode plates, the individual filter press units consisting of theouter frame, the grooved graphite separating and cathode plates and thediaphragm remain together and only the partially consumed ,anode platesneed be removed and replaced by new anode plates, and because of betteroperating conditions, the anode plates will last for ;5 to 6 years undernormal operation. Any disadvantage caused by replacement of the anodeplates every 5 to 6 years is more than olf-set by the ability to operatethe present cell at 30 to 35% higher current density.

Referring now to the drawings which illustrate a preferred embodiment ofthe invention:

FIG. l is a front view of a unit cell of the filter press assembly fromthe anode side;

FIG, 2 is a sectional top view of a unit cell along the line Y-Y of FIG.l;

FIG. 3 is a sectional top view of a unit cell along the line X-X of FIG.l;

FIG. 4 is a sectional side view of a unit cell along the line Z*Z ofFIG. 1;

FIG. 5 is a sectional side view of the unit cell along the line K-K ofFIG. l; and

FIG. 6 is an enlarged side view of the top of the unit cell as seen inFIG. 4.

As seen in FIGS. l, 2 and 3, the unit cell consists of an outer frame 1into which a plurality of grooved plates 2 are cemented to providesmooth surfaces 2a on the anodic side and grooved surfaces 2b on thecathodic side. To the smooth surface of the grooved plates 2, which onthe grooved side act as the cathode, there are bolted by means ofgraphite bolts 3 a number of smaller graphite plates 4 which act asanodes. The electrodes are spaced by an acid-resistant washer 3a made ofplexiglas or polyvinyl chloride, for example. The graphite anode plates4 have a smooth surface 4a on the inner side and a smooth or preferablygrooved surface 4b on the anodic side. If both sides of the anode plateare smooth, a space of a few mm. should be left between surface 4b andthe diaphragm to allow the gas bubbles formed to rise through the cell.

At the lower corners of frame 1 there are holes 5 and 6 through whichthe incoming hydrochloric acid passes around the length of the assembledcell. The passages 7 and 8 from the holes or openings 5 and 6respectively to the interior of the unit cell allow the concentratedhydrochloric acid supply of the unit cell to be maintained by theincoming hydrochloric acid owing in holes 5 and 6 while the spent acidllows from the top of each cell unitl One or both of holes 7 and 8 canbe used for draining the cell liquid in the case of a shut down anddisassembling of the cell.

At the upper corners, each frame r1 is provided with oblong openings 9and 10 connected by passages 9a and 10a with the interior of the unitcell and which when the cell is assembled provide channels from end toend of the assembled cell for the passage of hydrogen and chlorine gasesas well as spent electrolyte out of the cell.

If the incoming hydrochloric acid enters the cell unit in the anodicspace, lmiost of the spent electrolyte will leave with the chlorine gas.A small amount of acid will pass through the diaphragm to the cathodicspiace and will be removed as a mist or foam with the hydrogen gas. Iifthe incoming hydrochloric acid enters the cell unit in the cathodespace, most of the spent electrolyte will leave Iwith the hydrogen lgasand the small amount which passes through the diaphragm to the anodicspace will be removed as foam with the chlorine gas.

As can be seen in FIGS. 4, 5 and 6, each frame 1 has an integralextension 11 extending into the cell on the cathodic side with arabbeted edge 11a, a registering rabbeted groove 11b extends around .theentire frame, and an acid-resistant diaphragm 12 is cemented in thisrabbeted groove. The extension 11 acts as a gas separator to preventintermingling of the hydrogen and chlorine gases and the diaphnagms 12permanently cemented on the cathode side of the plates 2 serve toseparate the anode and lcaithode compartments. The hydrogen gas collectsin the space 13 and the chlorine gas collects in the space 14 and aredrawn olf through passages 9a and 10a, respectively. The diaphragms 12may be formed of woven polyvinyl chloride cloth as described in the F.I.A.T. Report No. 832 or any other suitable diaphragm material. In theframe 1 there is an opening closed by stopper 15, through which controlinstruments such as a thermometer or a manometer may be inserted whilethe electrolytic cell is operating. The stopper 15 may be a taperedpolyvinyl chloride plug and may be secured in place with polyvinylchloride cement.

On the smooth side 2a of the cathode plates 2 and about the graphitebolts 3 there is a coating 16 of gas and liquid impermeable materialsuch as a Bakelite varnish or resin. The coating 16 eliminateselectrolytiic conductivity through the pores of graphite plates 2 whichwould result in an apprecialble loss of current yield. The coating isalso aplpilied to the base of the opening for the bolts 3 to furthereliminate electrolytic conductivity. It is preferred to impregnate thecathode plates 2 by a series of partial impregnations to eliminate theelectrolytic conductivity.

While we have described a preferred embodiment of the invention, variousmodications of the apparatus of the present invention may be madewithout departing from the scope or spirit thereof, and it is to beunderstood that the invention is limi-ted only as defined in theappended claims.

We claim:

1. In an electrolytic cell of the iilter press type for the productionof O12 `and H2 by the electrolysis of hydrochloric acid, lilter pressframe members of chlorine and acid resistant material, hydrochloric acidinlets into said frame members, C12, H2 and spent acid outlets in saidframe members, the said inlets and outlets of one frame membercommunicating with similar inlets and outlets of adjacent frame members,cathode plate members secured in said frame members, said plate membershaving activated surfaces of a material selected from the groupconsisting of graphite and coke, integral grooves on one side of saidplate members forming the cathode plates, the other side of said platesbeing substantially smooth, an impregnating material sealing the smoothside of said cathode plates, `a diaphragm secured in said `frame memberson the grooved side of said cathode plates, and separate graphite anodeplates on the smooth side of said cathode plates, and means securingsaid anode plates to said cathode plates in electrical conductivecontact there- With.

2. In an electrolyftic cell of the iilter press type for the productionoff C12 and H2 by the electrolysis of hydrochloric acid, filter pressframe members of chlorine and acid resistant material, hydrochloric acidinlets in said frame members, O12, H2 and spent acid outlets in saidframe members, the said inlets and outlets of one frame membercommunicating with similar inlets and outlets of adjacent frame members,a projection of said frame members extending into the cell yas a gasseparating partition, cathode plate members secured in said framemembers, said plate members having activated surfaces of a materialselected from the group consisting of grap ite and coke, integralgrooves on one side of said plate members forming the cathode plates,the other side of said plates being substantially smooth, animpregnating material sealing the smooth side of said plates, adiaphragm secured in said frame members on the grooved side of saidplates, and separate graphite anode plates on the smooth side of saidcathode plates, and means removably securing said anode plates to saidcathode plates in electrical conductive contact therewith.

3. In an electrolytic cell of the filter press type for the productionof O12 and H2 by the electrolysis of hydrochloric acid, lilter pressframe members of chlorine and acid resistant material, hydrochloric acidinlets into said frame members, O12, H2 and spent acid outlets in saidframe members, the said inlets and outlets of one frame membercommunicating with similar inlets and outlets of adjacent trame members,a projection of said frame members extending into the cell as a gasseparating partition, cathode plate members secured in said framemembers, said plate members having activated surfaces of a materialselected from the group consistir-g of graphite and coke, integralgrooves on one side of said plate members forming the cathode plates,the other side of said plates being substantially smooth,` animpregnating material sealing the smooth side of said plates, a rabbetaround the interior of said frame members and across the bottoms of saidgas separating partitions, a diaphragm secured in said rabbet on thegrooved side of said plates,

and separate graphite anode plates on the smooth side of said cathodeplates, and means securing said anode plates to said cathode plates inelectrical conductive contact therewith.

4. In an electrolytic cell of the lilter press type for the productionof Cl2 and H2 by the electrolysis of hydrochloric acid, filter pressframe members of chlorine and acid resistant material, hydrochloric acidinlets into said frame members, O12, H2 and spent acid outlets in saidframe members, the said inlets and outlets of one frame membercommunicating with similar inlets and outlets of adjacent frame members,a projection of said frame members extending into the cell as a gasseperatiug partition, cathode plate members secured in said framemembers, said plate members having activated surfaces of a materialselected from the group consisting of graphite and coke, integralgrooves on one side of said plate members forming the cathode plates,the other side of said plates being substantially smooth, animpregnating material sealing the smooth side of said plates, adiaphragm secured in said frame members on the grooved side of saidplates, and separate grooved graphite anode plates on the smooth side ofsaid cathode plates, and graphite bolts securing said anode plates tosaid cathode plates in electrical conductive contact therewith.

5. In an electrolytic cell of the lilter press type tor the productionof C12 and H2 by the electrolysis of hydrochlonc acid, iilter pressframe members of chlorine and acid resistant material, hydrochloric acidinlets into said frame members, O12, H2 and sperrt acid outlets i-n saidframe members, the said inlets and outlets of one frame membercommunicating ywith similar inlets and outlets of adjacent framemembers, a projection of said frame members extending into the cell as agas separating partition, cathode plate members secured. in said framemembers, said plate members having activated surfaces of a materialselected from the group consisting of graphite and coke, integralgrooves on one side of said plate members forming the cathode plates,the other side of said plates being substantially smooth, animpregnating material sealing the smooth side of said plates, adiaphragm secured in said frame members on the grcoved side of saidplates, and separate smooth graphite anode plates on the `smooth side ofsaid cathode plates, and graphite bolts securing said anode plates tosaid cathode plates in electrical conductive contact therewith.

References Cited bythe Examiner UNITED STATES PATENTS 1,053,266 2/ 1913Barstow 204-256 1,907,818 5/ 1933 Hunter 20d-2,86 2,000,815 5/ 1935 Berl204-294 FOREIGN PATENTS 117,991 10/ 1927 Switzerland.

OTHER REFERENCES Heymann et al.: Germany application 1,054,430, printedApri19, 1959 (Ki 12 i2).

Rasche: German application 1,011,855, printed July 11, 1957 (Kl 12 h1).

JOHN H. MACK, Primary Examiner.

1. IN AN ELECTROLYTIC CELL OF THE FILTER PRESS TYPE FOR THE PRODUCTIONOF CL2 AND H2 BY THE ELECTROLYSIS OF HYDROCHLORIC ACID, FILTER PRESSFRAME MEMBERS OF CHLORINE AND ACID RESISTANT MATERIAL, HYDROCHLORIC ACIDINLETS INTO SAID FRAME MEMBERS, CL2 H2 AND SPENT ACID OUTLETS IN SAIDFRAME MEMBERS, THE SAID INLETS AND OUTLETS OF ONE FRAME MEMBERCOMMUNICATING WITH SIMILAR INLETS AND OUTLETS OF ADJACENT FRAME MEMBERS,CATHODE PLATE MEMBERS SECURED IN SAID FRAME MEMBERS, SAID PLATE MEMBERSHAVING ACTIVATED SURFACES OF A MATERIAL SELECTED FROM THE GROUPCONSISTING OF GRAPHITE AND COKE, INTEGRAL GROOVES ON ONE SIDE OF SAIDPLATE MEMBERS FORMING THE CATHODE PLATES, THE OTHER SIDE OF SAID PLATESBEING SUBSTANTIALLY SMOOTH, AN IMPREGNATING MATERIAL SEALING THE SMOOTHSIDE OF SAID